Nickel welded with tin possible? 2

[gandara]

We have some broken nickel parts and we only see the possibility to weld them as an interim solution with tin as per an electronic procedure.
A visible inspection and electric continuity test show the solution adequate now but will this contact be reliable in time in terms of environmental corrosion (95% humidity with 55 ºC maximum and a 30 ppm SO2 concentration?)

Thanks to anyone that can help in advance.

 

[JimMetalsCeramics]

If you are using pure tin, it is completely safe in humid environments. The only problem will come about if the part gets too cold (realistically below 10ºC). This can make the tin brittle.

 

[kenvlach]

The temperature and humidity are approximately optimal for creating 'whisker growth' from pure tin. I suggest using a 95% Sn solder (bal. either Bi, Sb, In, or Pb but not Cu) and giving it a coating of polyurethane.
See NASA's 'Tin Whisker Homepage' at

http://nepp.nasa.gov/whisker/

Also, "...the corrosion [of tin] is greatly accelerated by the presence of SO2, NO, NO2, and H2S in the air.."
This source mentions coating tin with wax for protection from humidities greater than 50%.

http://sul2.stanford.edu/byform/mailing-lists/cdl/2001/0391.html

Finally, why did the original part fail?

 

[gandara]

kenvlach, the parts broke due to a bad assembly in production. I was wondering if this kind of rework could work reliably. Wrt the polyurethane do you know the easiest way to apply it (maybe, spray,...)?
Thanks sincerely both for your so valuable help.

 

[JimMetalsCeramics]

The most straight-forward and reliable way to fix broken Ni parts is to furnace braze the pieces with Ag or Ni braze. This may be difficult. If you have to manually align the broken parts you'll have to bond with epoxy-based products or solder with a torch and flux. If the cross-section at the break is small enough, you can weld with a TIG torch and filler made of the mated materials.

 

[kenvlach]

A Sn-soldered joint will be weaker than solid Ni (electroformed?).

Yes, I would probably spray with a clear polyurethane (for protection and to be able to see the metal surface) from an aerosl can. You might want to do a search for other sprays suitable for protecting electrical devices. E.g., boo1 recommended a corrosion prevention spray 'Nylock' in thread307-61966.

 

[JimMetalsCeramics]

A solder joint is weaker but that's the trade-off.